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              使用 js 设置伪元素样式
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            <p>在开发一个ui编辑功能，因为使用的是Vue，最开始的时候是的方法是直接将styles放在dom元素上面，当遇到伪元素就无计可施了（切没发现JS直接获取DOM伪元素的方法）。</p>
            <p>于是就想能不能使用什么方法设置一段css样式</p>
            <h1 id="网络上的办法"><a href="#网络上的办法" class="headerlink" title="网络上的办法"></a>网络上的办法</h1>
            <p>我们可以这样：</p>
            <figure class="highlight js">
              <table>
                <tr>
                  <td class="gutter">
                    <pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre>
                  </td>
                  <td class="code">
                    <pre><span class="line"><span class="built_in">document</span>.styleSheets[<span class="number">0</span>].insertRule(<span class="string">&#x27;.test::before&#123;color:green&#125;&#x27;</span>,<span class="number">0</span>)<span class="comment">//chrome,firefox,ie9+,Edge,移动端</span></span><br><span class="line"><span class="comment">// 不推荐使用，实际使用过程中未生效</span></span><br><span class="line"><span class="built_in">document</span>.styleSheets[<span class="number">0</span>].addRule(<span class="string">&#x27;.test::before&#123;color:green&#125;&#x27;</span>,<span class="number">0</span>)<span class="comment">// IE 浏览器</span></span><br></pre>
                  </td>
                </tr>
              </table>
            </figure>
            <p>我们还可以这样：</p>
            <figure class="highlight js">
              <table>
                <tr>
                  <td class="gutter">
                    <pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre>
                  </td>
                  <td class="code">
                    <pre><span class="line"><span class="keyword">var</span> style=<span class="built_in">document</span>.createElement(<span class="string">&#x27;style&#x27;</span>);</span><br><span class="line">style.innerHTML=<span class="string">&quot;.test::before&#123;color:green&#125;&quot;</span>;</span><br><span class="line"><span class="built_in">document</span>.head.appendChild(style);</span><br></pre>
                  </td>
                </tr>
              </table>
            </figure>

            <p>我是在移动端使用的，实际上这两种方法都是可以的，但是我选择使用第二种，因为兼容性更好</p>
            <h1 id="解决兼容问题"><a href="#解决兼容问题" class="headerlink" title="解决兼容问题"></a>解决兼容问题</h1>
            <p>对于兼容问题MDN上面已经给出了一个答案 <a target="_blank" rel="noopener" href="https://developer.mozilla.org/zh-CN/docs/Web/API/CSSStyleSheet/insertRule">https://developer.mozilla.org/zh-CN/docs/Web/API/CSSStyleSheet/insertRule</a></p>
            <figure class="highlight js">
              <table>
                <tr>
                  <td class="gutter">
                    <pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br></pre>
                  </td>
                  <td class="code">
                    <pre><span class="line">(<span class="function"><span class="keyword">function</span>(<span class="params">Sheet_proto</span>)</span>&#123;</span><br><span class="line">  <span class="keyword">var</span> originalInsertRule = Sheet_proto.insertRule;</span><br><span class="line"></span><br><span class="line">  <span class="keyword">if</span> (originalInsertRule.length === <span class="number">2</span>)&#123; <span class="comment">// 2 个托管参数: (selector, rules)</span></span><br><span class="line">    Sheet_proto.insertRule = <span class="function"><span class="keyword">function</span>(<span class="params">selectorAndRule</span>)</span>&#123;</span><br><span class="line">      <span class="comment">// 首先，从规则中分离选择器</span></span><br><span class="line">      a: <span class="keyword">for</span> (<span class="keyword">var</span> i=<span class="number">0</span>, Len=selectorAndRule.length, isEscaped=<span class="number">0</span>, newCharCode=<span class="number">0</span>; i !== Len; ++i) &#123;</span><br><span class="line">        newCharCode = selectorAndRule.charCodeAt(i);</span><br><span class="line">        <span class="keyword">if</span> (!isEscaped &amp;&amp; (newCharCode === <span class="number">123</span>)) &#123; <span class="comment">// 123 = &quot;&#123;&quot;.charCodeAt(0)</span></span><br><span class="line">          <span class="comment">// 其次，找到花括号</span></span><br><span class="line">          <span class="keyword">var</span> openBracketPos = i, closeBracketPos = -<span class="number">1</span>;</span><br><span class="line"></span><br><span class="line">          <span class="keyword">for</span> (; i !== Len; ++i) &#123;</span><br><span class="line">            newCharCode = selectorAndRule.charCodeAt(i);</span><br><span class="line">            <span class="keyword">if</span> (!isEscaped &amp;&amp; (newCharCode === <span class="number">125</span>)) &#123; <span class="comment">// 125 = &quot;&#125;&quot;.charCodeAt(0)</span></span><br><span class="line">              closeBracketPos = i;</span><br><span class="line">            &#125;</span><br><span class="line">            isEscaped ^= newCharCode===<span class="number">92</span>?<span class="number">1</span>:isEscaped; <span class="comment">// 92 = &quot;\\&quot;.charCodeAt(0)</span></span><br><span class="line">          &#125;</span><br><span class="line"></span><br><span class="line">          <span class="keyword">if</span> (closeBracketPos === -<span class="number">1</span>) <span class="keyword">break</span> a; <span class="comment">// No closing bracket was found!</span></span><br><span class="line">            <span class="comment">/*else*/</span> <span class="keyword">return</span> originalInsertRule.call(</span><br><span class="line">            <span class="built_in">this</span>, <span class="comment">// 想要改变的样式表</span></span><br><span class="line">            selectorAndRule.substring(<span class="number">0</span>, openBracketPos), <span class="comment">// 选择器</span></span><br><span class="line">            selectorAndRule.substring(closeBracketPos), <span class="comment">// 规则</span></span><br><span class="line">            <span class="built_in">arguments</span>[<span class="number">3</span>] <span class="comment">// 插入的索引</span></span><br><span class="line">          );</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        <span class="comment">// Works by if the char code is a backslash, then isEscaped</span></span><br><span class="line">        <span class="comment">// gets flipped (XOR-ed by 1), and if it is not a backslash</span></span><br><span class="line">        <span class="comment">// then isEscaped gets XORed by itself, zeroing it</span></span><br><span class="line">        isEscaped ^= newCharCode===<span class="number">92</span>?<span class="number">1</span>:isEscaped; <span class="comment">// 92 = &quot;\\&quot;.charCodeAt(0)</span></span><br><span class="line">      &#125;</span><br><span class="line">      <span class="comment">// Else, there is no unescaped bracket</span></span><br><span class="line">      <span class="keyword">return</span> originalInsertRule.call(<span class="built_in">this</span>, selectorAndRule, <span class="string">&quot;&quot;</span>, <span class="built_in">arguments</span>[<span class="number">2</span>]);</span><br><span class="line">    &#125;;</span><br><span class="line">  &#125;</span><br><span class="line">&#125;)(CSSStyleSheet.prototype);</span><br></pre>
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                </tr>
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            <p>这段代码应该放在何处才能生效，可以自己思考一下，至于是否选择这个兼容方案大家自己选择。</p>
            <h1 id="插入顺序问题"><a href="#插入顺序问题" class="headerlink" title="插入顺序问题"></a>插入顺序问题</h1>
            <p>这两个方法因为都是插入了一段CSS代码，CSS在解析的时候会根据引入的先后顺序和权重值来解析，导致我们插入的代码被覆盖，虽然第二种方法是插入到了head最后，一定程度上避免了这种问题，但是不能完全解决这种隐患 </p>
            <p>解决办法就是当发现或者凭经验判断样式未生效，需要在当前功能执行之后在head最后重新添加对应的CSS代码</p>
            <h1 id="删除无效CSS代码"><a href="#删除无效CSS代码" class="headerlink" title="删除无效CSS代码"></a>删除无效CSS代码</h1>
            <ul>
              <li>对于第一种方法<br><a target="_blank" rel="noopener" href="https://developer.mozilla.org/zh-CN/docs/Web/API/CSSStyleSheet/deleteRule">CSSStyleSheet.deleteRule()</a> 移除指定的CSS代码</li>
              <li>对于第二种方法<br>可以在创建<code>style</code>的时候设置一个id，可以使用 <code>document.styleSheet</code>来匹配id，可以是移除dom或者使用 <a target="_blank" rel="noopener" href="https://developer.mozilla.org/zh-CN/docs/Web/API/CSSStyleSheet/deleteRule">CSSStyleSheet.deleteRule()</a>移除指定的CSS代码</li>
            </ul>
            <h1 id="参考地址"><a href="#参考地址" class="headerlink" title="参考地址"></a>参考地址</h1>
            <p><a target="_blank" rel="noopener" href="https://developer.mozilla.org/zh-CN/docs/Web/API/Document/styleSheets">DocumentOrShadowRoot.styleSheets</a><br><a target="_blank" rel="noopener" href="https://developer.mozilla.org/zh-CN/docs/Web/API/Document/styleSheetSets">Document.styleSheetSets</a><br><a target="_blank" rel="noopener" href="https://developer.mozilla.org/zh-CN/docs/Web/API/CSSStyleSheet">CSSStyleSheet</a><br><a target="_blank" rel="noopener" href="https://developer.mozilla.org/zh-CN/docs/Web/API/CSSStyleSheet/insertRule#notes">CSSStyleSheet.insertRule()</a><br><a target="_blank" rel="noopener" href="https://developer.mozilla.org/zh-CN/docs/Web/API/Document/createElement">Document.createElement()</a></p>
            <p><a target="_blank" rel="noopener" href="https://zhuanlan.zhihu.com/p/63760451">使用 js 设置伪元素样式</a><br><a target="_blank" rel="noopener" href="https://www.w3school.com.cn/js/js_htmldom_css.asp">JavaScript HTML DOM - 改变 CSS</a><br><a target="_blank" rel="noopener" href="http://c.biancheng.net/view/6023.html">JS addRule()和insertRule()方法：添加CSS样式</a><br><a target="_blank" rel="noopener" href="https://www.cnblogs.com/ypppt/p/13039786.html">如何使用JS操纵伪元素</a><br><a target="_blank" rel="noopener" href="https://blog.csdn.net/weixin_43079157/article/details/103044064">Js动态加载CSS样式文件的2种方法</a></p>

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